In industrial automation, serial bus systems are used for communication between the sensor/actuator level and the control level. Serial bus systems of this kind, also called field buses in industrial automation, are subject to high communication requirements in relation to transmission speed and freedom from error. The sensor/actuator level and the control level have process states, i.e. sensor data and actuator commands, transmitted between them in real time. This data interchange normally takes place cyclically. Besides cyclic data interchange of this kind, acyclic data are also transmitted between the sensor/actuator level and the control level, however, for example in order to transfer parameters to the devices of the sensor/actuator level or to query status data.
The data interchange on a field bus is normally executed on the basis of the master/slave principle. The active bus subscribers, generally the control nodes of the control level, are in possession of bus access authorization and determine the data transfer. By contrast, the sensors and actuators are usually the passive bus subscribers. They are not provided with bus access authorization, i.e. they can only acknowledge received information signals or transmit information signals to a control node upon a request from the latter.
The information signals are sent by the control nodes generally in the form of data messages that are made up of control data and useful data, the data messages preferably being designed on the basis of the Ethernet standard. The sensors and actuators connected to the bus process the useful data intended for the respective device that are in the data messages fed onto the bus by the control node.
Field bus systems having a master/slave structure are normally, in order to avoid complex wiring, designed using ring topology, with all network subscribers being connected to a ring-shaped data path. A data message produced by a control node is fed into the data path by the control node and successively transits through the further network subscribers serially connected to the data path so as then to be received by the control node again.
In order to allow high data transmission rates in the network, a full-duplex mode is preferably executed. In this case, the ring-shaped data path is made up of an outbound route and an inbound route that connects the network subscribers and thus permits simultaneous sending and receiving by the network subscribers. The network is designed such that starting from the control node the network subscribers form a chain with a head subscriber as a termination, with the ring-shaped data path transiting through the network subscribers on both the outbound route and the inbound route. The network subscribers can then perform data interchange with data messages that are fed onto the data path by the control node both on the outbound route and on the inbound route.
Owing to the increasing data transmission rates—in Ethernet networks, a data transmission rate of 1 Gbit/s is increasingly being used instead of the usual data transmission rate of 100 Mbit/s—the delay time for the data messages through the network subscribers is gaining ever greater significance for data interchange speed between the sensor/actuator level and the control level.
In industrial automation, the control using data interchange via networks is normally effected such that the control node cyclically performs control processes in order to ascertain output data for the sensor/actuator level on the basis of input data of the sensor/actuator level. Following termination of a control process cycle, the control node sends the output data in the form of data messages, wherein the network subscribers read the output data associated with the respective network subscriber and then use these output data to execute a local network subscriber process. The data ascertained with these local network subscriber processes are then returned to the control node by the network subscribers and subsequently used as input data for the next control process cycle by the control node.
In this case, the output data from the data messages that are output by the control node are read in by the network subscribers on the outbound route of the data path. The input data provided by the network subscribers are read into this or a further data message sent by the control node on the inbound path and fed back to the control node. Since the transit time through the network subscribers, during which the data are read out and in, cannot be shortened to the same extent as the data transmission rate in the network is increased, the shortest cycle time for a control process by the control node is increasingly determined by the circulation time for the data messages on the ring-shaped data path, which circulation time is obtained on the basis of the transit time through the network subscribers.